The present application generally relates to optical imaging, and more particularly, to compensation for optical distortion in optical systems.
An optical imaging system having a defined optic axis can produce a geometrically-distorted image of an extended object with parts located away from the optic axis. Such distortion is in part caused by deviation of object points from the paraxial conditions. The amount of the distortion increases with the distance of an image point from the optic axis. One way to understand such distortion is that the magnification of the optical system varies across the imaging field. Other defects and imperfections in the optical components such as imaging lenses of the optical system also contribute to the total distortion of the image.
Two examples of some commonly-encountered geometrical distortions include pincushion distortion and barrel distortion as illustrated in FIG. 1. When magnification increases with the image point distance from the optic axis, the pincushion distortion occurs in which the image of a square has concave sides. In the opposite, when the magnification decreases with the image point distance from the optic axis, the barrel distortion occurs in which the image of a square has convex sides.
The distortion of an optical system can be reduced or compensated optically in the optical path of the system by controlling the physical attributes of one or more optical elements such as indices of refraction and physical dimensions of lenses in the system. One method uses composite lenses to achieve certain distortion correction. One limitation of such an approach is the complexity of the optical design. Manufacturing of such composite lens system often requires sophisticated fabrication and can be expensive. Hence, it may not be suitable for many mass-produced optical systems used in various applications such as digital cameras and camcorders.
Another limitation of the optical approach is that an optical design for compensating one type of geometric distortion may worsen one or more different aberrations. For example, a composite lens system designed for correcting either pincushion or barrel distortion may increase spherical aberration, chromatic aberration, coma, astigmatism, or Petzval curvature.
Therefore, it is desirable to find a different approach to correct geometrical distortions of an optical system.
In recognition of the above and other limitations of optical correction of distortions, the present disclosure provides a non-optical technique to correct the total distortion caused by the imaging optics at the image plane. This is accomplished by using a specially designed sensor array at the image plane to detect the distorted image and to reconstruct an image of the object with a substantially reduced distortion by using pixel signals.
An imaging device according to one embodiment includes imaging optics to receive radiation from an object and to project an image of the object to an image plane, and a semiconductor sensor array located adjacent or at the image plane to receive the image. The imaging optics produces a distortion having a distortion profile at least in a portion of said image. The sensor array has a plurality of sensing pixels arranged relative to one another to substantially reduce the distortion profile produced by the imaging optics so as to produce pixel signals indicative of the object with a reduced distortion.
The sensing pixels may be arranged in columns and rows and at least a portion of the columns and rows are curved in a manner determined by the distortion profile. In one implementation, each sensing pixel has a photosensitive area whose size depends on its position in the sensor array according to the distortion profile to correct a distortion in an intensity distribution associated with the distortion of the imaging optics. In another implementation, the sensing pixels each have photosensitive areas that are substantially identical to one another. Accordingly, a processing circuit is provided to modify the pixel signals to correct the distortion in the intensity distribution.
Further, an intensity distortion caused by the vignetting effect of the imaging optics and the sensor array may also be compensated for by changing the size of each photosensitive area or modifying output signal of each pixel according to its position in the sensor array.
A method for correcting optical distortion in an imaging system is also disclosed. First, a distortion profile of an imaging optics at a plane is determined. Then, a sensor array of sensing pixels is formed by arranging the sensing pixels in curved rows and columns based on the distortion profile. Next, the sensor array is placed adjacent or at the plane to convert an image produced by the imaging optics into pixel signals that represent an electronic version of the image with a reduced amount of distortion.